A California woman is suing Ulta Beauty, the suburban Chicago-based cosmetics retailer, for allegedly reselling used makeup as new to unsuspecting consumers.

The lawsuit, filed last month in a federal court in Chicago, seeks class-action status for what it claims is Ulta’s “widespread and surreptitious practice” of repackaging returned cosmetics and mixing them back on the shelf to sell at full price with unblemished products.

“Every customer who has purchased cosmetics at Ulta since this practice began was put at risk of unwittingly purchasing used, unsanitary cosmetics and this risk reduces the desirability and value of all cosmetics sold by Ulta,” the lawsuit alleges.

The lawsuit was brought by Kimberley Laura Smith-Brown, a Los Angeles woman who purchased dozens of cosmetic items, including eye liner, mascara and, most recently, lip balm, at an Ulta store over the past six months — before reports of the alleged practice of reselling used makeup surfaced last month on social media.

“We are aware of the lawsuit, and intend to vigorously defend against the allegations,” Ulta Beauty spokeswoman Karen Twigg May said Friday in an email. “Our policies, training and procedures are aimed at selling only the highest-quality new products in our stores and online. The health and safety of Ulta Beauty guests is a top priority, and we strive to consistently deliver an optimal experience every time they shop with us.”

Launched in 1990, Bolingbrook-based Ulta bills itself as the largest beauty retailer in the U.S., selling cosmetic, fragrance, skin care, hair care and other products through 1,058 retail stores and its website. The company had a net income of about $410 million on sales of nearly $4.9 billion in 2016, according to its most recent annual report.

The alleged practice of reselling returned cosmetics came to light on Jan. 9, when Twitter user @fatinamxo, who claimed to be a former Ulta employee, posted several messages about her experiences with reselling used products:

Her Twitter feed lit up with posts from others who claimed to be Ulta employees, with some supporting her contention and others rejecting it. Meanwhile, customers mostly expressed shock and displeasure over the alleged practice, coloring their comments with expressive GIFs and emojis.

Efforts to reach @fatinamxo Friday through Twitter were unsuccessful.

Smith-Brown and her attorneys are seeking to form a class that would extend to everyone who purchased cosmetics from Ulta Beauty retail locations since every sale is “tainted with the possibility that the customer is purchasing used, dirty cosmetics,” according to the lawsuit.

The lawsuit raises but is not based on the possible health concerns of reusing makeup. Instead, it is suing Ulta for “unjust enrichment” by reselling used products as new to unaware consumers.

In addition to seeking certification as a class action, the lawsuit asks for undisclosed damages and other relief.